1. Field of the Invention
The invention concerns an electrostatic spraying installation for a conductive liquid coating product such as a water-based paint or a metallic paint. The invention is more particularly concerned with an installation of this kind comprising at least one isolator having a mobile pipe member inserted into the circuit for distributing the coating product to provide the necessary electrical insulation between the parts of the distribution circuit at ground potential and those at a high voltage during spraying of the coating product. The improvement in accordance with the invention enables the operation of an isolator of this kind to be improved and its reliability to be augmented.
2. Description of the Prior Art
Installations of the above kind comprising one or more mobile pipe member isolators for simultaneously interrupting the flow of the coating product and electrically insulating the downstream parts are known. U.S. Pat. No. 4,313,475 describes one such installation in which the isolator is actuated by a ram and comprises two pipe members each provided with a shut-off valve, one of the pipe members being movable towards the other. The valves are opened by opening one of them which causes the other to be opened. A system of this kind is attractive at first sight because both valves can be operated by the ram actuating one valve. However, these systems are not very reliable when conveying conductive liquid coating products such as water-based paints. These coating products are aqueous dispersions of organic resin mixed with solid mineral charges and possibly with metallic pigments. These dispersions are fragile, abrasive and oxidizing, the suspension agent being demineralized water. If the suspension is destroyed, in other words if the aqueous phase is separated from the less fluid resinous phase, the latter adheres to the walls or to the mechanical parts and is much more difficult to clean off. For example, it is then necessary to use a solvent for the resin itself rather than a simple rinsing product such as water. This problem occurs notably in the interstices between the moving parts of an isolator of the kind described above, especially the mating faces, the shut-off balls and their seat. All these parts are quickly soiled and eroded and after some time sealing is lost. It may therefore be necessary to use different cleaning products. One is an inexpensive rinsing product that is not very aggressive, such as water. It is merely capable of entraining and diluting the residues of the coating product. The other, more costly and more aggressive, is a solvent capable not only of rinsing but also of stripping and dissolving residues that have separated out and become deposited on the walls of the pipe and on moving parts.
Another type of electrostatic spraying installation for conductive liquid coating products provides electrical insulation by means of a simple section of insulative pipe, its length being sufficient to "withstand" the high voltage. This pipe section is controlled by valves which admit the coating product to fill the auxiliary storage tank and then the rinsing product and the compressed air to clean the insulative pipe member and to dry it thoroughly so that it is able to function as an electrical insulator. A system of this kind is described in French patent application No 2 572 662, for example. It is difficult to use and requires excessively long cleaning and especially drying cycles which are difficult to control in the automobile industry in which coating product changes are frequent and have to be accomplished in a very short time determined by the rate of production of the objects to be coated.
Finally, in another known type installation electrical insulation is obtained by means of an isolator consisting of an insulative material pipe section inserted into the distribution circuit and provided internally with a mobile scraper member. The displacement of this mobile member cleans the inside wall of said pipe section to make it sufficiently insulative.
The invention embodies a new concept for providing quickly electrical insulation between the two parts of the electrostatic spraying installation. The basic idea of the invention is to use a mobile member isolator but to provide the necessary means in the installation for cleaning and purging the isolator before it is opened each time. Note that cleaning and purging the isolator do not imply complete drying through the prolonged circulation of compressed air. This significantly improves the reliability of a mobile member isolator of this kind without significantly increasing the connection-disconnection times.